Chemistry · High school and introductory college chemistry

Chemical Bonds and the Periodic Table Study Guide

Atoms bond to reach a more stable arrangement of valence electrons: in ionic bonds electrons are transferred between a metal and a nonmetal, and in covalent bonds electrons are shared between nonmetals. The periodic table predicts which kind of bond forms, because an element’s group tells you how many valence electrons it has and its electronegativity shows how strongly it pulls on shared electrons. Most questions ask you to predict a bond type or a charge from an element’s position.

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Key terms

Valence electrons: Electrons in an atom’s outermost shell, which take part in bonding.
Ion: An atom or group of atoms with an electric charge because it has gained or lost electrons.
Cation: A positively charged ion, formed when an atom loses electrons. Metals usually form cations.
Anion: A negatively charged ion, formed when an atom gains electrons. Nonmetals usually form anions.
Ionic bond: Attraction between oppositely charged ions, formed when electrons are transferred from a metal to a nonmetal.
Covalent bond: Bond formed when atoms share one or more pairs of electrons. It usually forms between nonmetals.
Polar covalent bond: A covalent bond in which electrons are shared unequally, so one end is slightly negative and the other slightly positive.
Nonpolar covalent bond: A covalent bond in which electrons are shared equally, as between two atoms of the same element.
Metallic bond: Attraction between metal cations and a sea of shared, freely moving electrons.
Electronegativity: An atom’s ability to attract shared electrons in a bond.
Periodic table group: A vertical column of elements that share similar chemical properties. In the main groups it also tells you the number of valence electrons.
Period: A horizontal row of the periodic table. The period number is the number of electron shells.
Octet rule: The tendency of atoms to gain, lose or share electrons until they have eight in the outer shell.
Noble gases: Group 18 elements, with full outer shells, which rarely form bonds.

The periodic table as a map

•Group 1 (alkali metals) have 1 valence electron and form +1 ions.
•Group 2 elements have 2 valence electrons and form +2 ions.
•Group 17 (halogens) have 7 valence electrons and form −1 ions.
•Group 18 (noble gases) have full outer shells and rarely react.
•Electronegativity generally increases across a period and decreases down a group.

Ionic versus covalent

•Ionic: a metal and a nonmetal; electrons are transferred; the compound is a lattice of ions with a high melting point and conducts electricity when melted or dissolved.
•Covalent: two nonmetals; electrons are shared; the result is molecules, usually with lower melting points that do not conduct electricity as a solid.

Predicting the type of bond

Compare the electronegativity of the two atoms. As a common rule of thumb, a difference above about 1.7 is ionic, about 0.4 to 1.7 is polar covalent, and below about 0.4 is nonpolar covalent. Textbooks use slightly different cutoffs, so use the one your class does.

•NaCl (table salt) is ionic.
•H₂O (water) has polar covalent bonds.
•O₂ (oxygen gas) has a nonpolar covalent bond.

Writing ionic formulas

The charges in an ionic compound must add up to zero. Na⁺ and Cl⁻ combine one to one as NaCl. Mg²⁺ needs two Cl⁻, so magnesium chloride is MgCl₂.

Common mistakes

Mistake: In an ionic bond electrons are shared.
In an ionic bond electrons are transferred. Sharing is what covalent bonds do.
Mistake: Ionic compounds are made of molecules.
Ionic compounds form a repeating lattice of ions, so their formulas show the simplest ratio, not a molecule.
Mistake: Metals gain electrons to become anions.
Metals usually lose electrons and become cations. Nonmetals gain electrons and become anions.
Mistake: The group number tells you the number of shells.
The period (row) number gives the number of shells. For main-group elements, the group tells you the valence electrons.
Mistake: Covalent bonds always share electrons equally.
They do only between identical atoms. Between different atoms the sharing is usually unequal, which makes the bond polar.

Common questions

What is the difference between ionic and covalent bonds?↓

In an ionic bond, one atom transfers electrons to another and the resulting ions attract each other. In a covalent bond, atoms share electrons. Ionic bonds usually form between metals and nonmetals, and covalent bonds between nonmetals.

How can I tell which bond forms from the periodic table?↓

Look at where the elements are. A metal on the left with a nonmetal on the right usually forms an ionic bond. Two nonmetals usually form a covalent bond. The electronegativity difference gives a more precise answer.

What are valence electrons and how do I find them?↓

They are the electrons in the outermost shell and they do the bonding. For main-group elements, the group number tells you: group 1 has one, group 2 has two, and group 17 has seven.

What is the electronegativity trend?↓

Electronegativity generally increases from left to right across a period and decreases from top to bottom down a group. Fluorine is the most electronegative element.

What is the best way to memorize the periodic table groups?↓

Do not memorize the whole table. Learn the families that matter (alkali metals, alkaline earth metals, halogens, noble gases) and what charge each forms, then quiz yourself on them.